1. Field of the Invention
The present invention relates to an environmentally compatible and solderable wire enamel for producing solderable coatings on wires, on the basis of polyurethanes in solution in hydrocarbons.
2. Discussion of Background Information
Polyurethanes as electrically insulating coating materials based on hydroxyl-containing polyesters and blocked isocyanates are known in large numbers and described in, for example, DE 195 71 57, DE 100 51 392 and DE 195 07 942. In the context of their use as wire-coating materials, electrically insulating coating materials of this kind are notable in particular for their good insulating properties. Another advantage they possess is that of being solderable. When immersed into a solder bath which has been heated to elevated temperatures, solderable insulated wires undergo destruction of the insulating layer to expose the bare metal of the conductor, which is therefore directly accessible for electrically conducting connections. The solderability is attributable to the presence of urethane groups in the enamel film.
State of the art are polyurethane wire enamels composed of hydroxyl-comprising polyesters, blocked isocyanates, catalysts, auxiliaries, and a complex solvent mixture. Principal solvents are cresolic solvents, which are blended with aromatic hydrocarbons. Cresols and xylenols are solvents which are available technically. Since they constitute distillation cuts, the isomer composition varies from one grade to another. A considerable disadvantage is that all cresolic solvents are toxic and carry an extremely unpleasant odor.
These wire enamels are processed on commercial wire enameling lines, in which the solvent mixture is evaporated off and the binder is cured. The solvent removed by evaporation is burnt on the enameling line and used as part of the heating for the curing oven.
DE 27 18 898 and EP 0 055 085 disclose coating materials comprising polyesterimides which comprise blocked isocyanates. One of the characteristics of the coating materials described therein, however, is that they are not solderable.
Cresol-free wire enamels are known, the cresolic solvent being replaced by a different solvent. For example, methyldiglycol, ethyldiglycol, gamma-butyrolactone (Lienert in “Lösemittel für kresolfreie Drahtlacke”, 8th Conference on Electrical Insulating Systems, Hamburg 1989), alkylene carbonates (EP 0 502 858), and many others may be used. As a result of the fact that the alternative solvents usually have a lower calorific value than their cresolic counterparts, there is a reduction in energy to the enameling machine.